Method of modifying ceramic composite bodies by a post-treatment process and articles produced thereby

ABSTRACT

This invention relates generally to a novel method of manufacturing a composite body, such as a ZrB 2  -ZrC-Zr (optional) composite body, by utilizing a post-treatment process and to the novel products made thereby. More particularly, the invention relates to a method of modifying a composite body comprising one or more boron-containing compounds (e.g., a boride or a boride and a carbide) which has been made by the reactive infiltration of a molten parent metal into a bed or mass containing boron carbide, and optionally one or more inert fillers, to form the body.

This is a continuation of copending application Ser. No. 07/296,239filed on Jan. 12, 1989, now abandoned.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of U.S. Pat. No. 4,915,736,which issued on Apr. 10, 1990, from U.S. patent application Ser. No.137,382, filed Dec. 23, 1987, in the names of Terry Dennis Claar andGerhard Hans Schiroky, and entitled "A Method of Modifying CeramicComposite Bodies by a Carburization Process and Articles ProducedThereby", the subject matter of which is herein incorporated byreference.

FIELD OF THE INVENTION

This invention relates generally to a novel method of manufacturing acomposite body, such as a ZrB₂ -ZrC-Zr (optional) composite body, byutilizing a post-treatment process and to the novel products madethereby. More particularly, the invention relates to a method ofmodifying a composite body comprising one or more boron-containingcompounds (e.g., a boride or a boride and a carbide) which has been madeby the reactive infiltration of a molten parent metal into a bed or masscontaining boron carbide, and optionally one or more inert fillers, toform the body.

BACKGROUND OF THE INVENTION

In recent years, there has been an increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the relative superiority of ceramics,when compared to metals, with respect to certain properties, such ascorrosion resistance, hardness, wear resistance, modulus of elasticityand refractory capabilities.

However, a major limitation on the use of ceramics for such purposes isthe feasibility and cost of producing the desired ceramic structures.For example, the production of ceramic boride bodies by the methods ofhot pressing, reaction sintering, and reaction hot pressing is wellknown. While there has been some limited success in producing ceramicboride bodies according to the above-discussed methods, there is still aneed for a more effective and economical method to prepare denseboride-containing materials.

In addition, a second major limitation on the use of ceramics forstructural applications is that ceramics generally exhibit a lack oftoughness (i.e., damage tolerance, or resistance to fracture). Such lackof toughness tends to result in sudden, easily induced, catastrophicfailure of ceramics in applications involving rather moderate tensilestresses. This lack of toughness tends to be particularly common inmonolithic ceramic boride bodies.

One approach to overcome the above-discussed problem has been theattempt to use ceramics in combination with metals, for example, ascermets or metal matrix composites. The objective of this known approachis to obtain a combination of the best properties of the ceramic (e.g.,hardness and/or stiffness) and the best properties of the metal (e.g.,ductility). While there has been some general success in the cermet areain the production of boride compounds, there still remains a need formore effective and economical methods to prepare boride-containingmaterials.

DISCUSSION OF RELATED PATENT APPLICATIONS

Many of the above-discussed problems associated with the production ofboride-containing materials have been addressed in co-owned U.S. patentapplication Ser. No. 073,533, now abandoned, filed in the names of DannyR. White, Michael K. Aghajanian and T. Dennis Claar, on Jul. 15, 1987,and entitled "Process for Preparing Self-Supporting Bodies and ProductsMade Thereby". The subject matter of application Ser. No. 073,533(hereinafter referred to as Application '533) is herein expresslyincorporated by reference.

The following definitions were used in Application '533 and shall applyto the instant application as well.

"Parent metal" refers to that metal (e.g., zirconium) which is theprecursor for the polycrystalline oxidation reaction product, that is,the parent metal boride or other parent metal boron compound, andincludes that metal as a pure or relatively pure metal, a commerciallyavailable metal having impurities and/or alloying constituents therein,and an alloy in which that metal precursor is the major constituent; andwhen a specific metal is mentioned as the parent metal (e.g. zirconium),the metal identified should be read with this definition in mind unlessindicated otherwise by the context.

"Parent metal boride" and "parent metal boro compounds" mean a reactionproduct containing boron formed upon reaction between boron carbide andthe parent metal and includes a binary compound of boron with the parentmetal as well as ternary or higher order compounds.

"Parent metal carbide" means a reaction product containing carbon formedupon reaction of boron carbide and parent metal.

Briefly summarizing the disclosure of Application '533, self-supportingceramic bodies are produced by utilizing a parent metal infiltration andreaction process (i.e., reactive infiltration) in the presence of aboron carbide. Particularly, a bed or mass of boron carbide isinfiltrated by molten parent metal, and the bed may be comprisedentirely of boron carbide, thus resulting in a self-supporting bodycomprising one or more parent metal boron-containing compounds, whichcompounds include a parent metal boride or a parent metal boro carbide,or both, and typically also may include a parent metal carbide. It isalso disclosed that the mass of boron carbide which is to be infiltratedmay also contain one or more inert fillers mixed with the boron carbide.Accordingly, by combining an inert filler, the result will be acomposite body having a matrix produced by the reactive infiltration ofthe parent metal, said matrix comprising at least one boron-containingcompound, and the matrix may also include a parent metal carbide, thematrix embedding the inert filler. It is further noted that the finalcomposite body product in either of the above-discussed embodiments(i.e., filler or no filler) may include a residual metal as at least onemetallic constituent of the original parent metal.

Broadly, in the disclosed method of Application '533, a mass comprisingboron carbide is placed adjacent to or in contact with a body of moltenmetal or metal alloy, which is melted in a substantially inertenvironment within a particular temperature envelope. The molten metalinfiltrates the boron carbide mass and reacts with the boron carbide toform at least one reaction product. The boron carbide is reducible, atleast in part, by the molten parent metal, thereby forming the parentmetal boron-containing compound (e.g., a parent metal boride and/or borocompound under the temperature conditions of the process). Typically, aparent metal carbide is also produced, and in certain cases, a parentmetal boro carbide is produced. At least a portion of the reactionproduct is maintained in contact with the metal, and molten metal isdrawn or transported toward the unreacted boron carbide by a wicking ora capillary action. This transported metal forms additional parentmetal, boride, carbide, and/or boro carbide and the formation ordevelopment of a ceramic body is continued until either the parent metalor boron carbide has been consumed, or until the reaction temperature isaltered to be outside of the reaction temperature envelope. Theresulting structure comprises one or more of a parent metal boride, aparent metal boro compound, a parent metal carbide, a metal (which, asdiscussed in Application '533, is intended to include alloys andintermetallics), or voids, or any combination thereof. Moreover, theseseveral phases may or may not be interconnected in one or moredimensions throughout the body. The final volume fractions of theboron-containing compounds (i.e., boride and boron compounds),carbon-containing compounds, and metallic phases, and the degree ofinterconnectivity, can be controlled by changing one or more conditions,such as the initial density of the boron carbide body, the relativeamounts of boron carbide and parent metal, alloys of the parent metal,dilution of the boron carbide with a filler, temperature, and time.

The typical environment or atmosphere which was utilized in Application'533 was one which is relatively inert or unreactive under the processconditions. Particularly, it was disclosed that an argon gas, or avacuum, for example, would be suitable process atmospheres. Stillfurther, it was disclosed that when zirconium was used as the parentmetal, the resulting composite comprised zirconium diboride, zirconiumcarbide, and residual zirconium metal. It was also disclosed that whenaluminum parent metal was used with the process, the result was analuminum boro carbide such as Al₃ B₄₈ C₂, AlB₁₂ C₂ and/or AlB₂₄ C₄, withaluminum parent metal and other unreacted unoxidized constituents of theparent metal remaining. Other parent metals which were disclosed asbeing suitable for use with the processing conditions included silicon,titanium, hafnium, lanthanum, iron, calcium, vanadium, niobium,magnesium, and beryllium.

Thus, Application '533 discloses a novel process, and novel bodiesresulting from the process, which overcomes many of the deficiencies ofthe prior art discussed above, thus satisfying a long-felt need.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the foregoing and isan improvement over the prior art.

The invention provides a method for modifying the resultant amount ofparent metal present in a composite body. Particularly, in a firstpreferred embodiment, the amount of parent metal can be modified orcontrolled by exposing the composite body (i.e., the residual parentmetal in the composite body) to a carburizing environment (e.g., eithera gaseous carburizing species or a solid carbon material) which modifiesthe composition of the residual parent metal, thus modifying theproperties of the residual parent metal. Moreover, the properties of theresultant composite body can also be modified. Parent metals such aszirconium, titanium, and hafnium are well suited to be treated by thecarburizing processes according to the present invention.

In a second preferred embodiment, the amount of parent metal can bemodified or controlled by exposing the composite body (i.e., theresidual parent metal in the composite body) to a boriding environment(e.g., by heating a composite body containing unreacted parent metal ina substantially inert atmosphere while contacting (e.g., being embeddedin) a mass comprising a boron source). The metal remaining in thecomposite reacts with the boron source to form a parent metal boride,thus modifying the properties of the resultant composite. The metalcontent of the composite can be controlled to provide a compositecomprising about 0 to about 2 volume percent parent metal. Parent metalssuch as zirconium, titanium, and hafnium are well suited to be treatedby the boriding processes according to the present invention.

In a third preferred embodiment, the amount of parent metal can bemodified or controlled by exposing the composite body (i.e., theresidual parent metal in the composite body) to a nitriding environment(e.g., by heating a composite body containing unreacted parent metal ina substantially inert atmosphere while contacting (e.g., being embeddedin) a mass comprising a nitrogen source). The metal remaining in thecomposite reacts with the nitride source to form a parent metal nitride,thus modifying the properties of the resultant composite. The metalcontent of the composite can be controlled to provide a compositecomprising about 0 to about 2 volume percent parent metal. Parent metalssuch as zirconium, titanium, and hafnium are well suited to be treatedby the nitriding processes according to the present invention. Thisapplication refers primarily to ZrB₂ -ZrC-Zr composite bodies,hereinafter referred to as "ZBC" composite bodies. However, it should beunderstood that while specific emphasis has been placed upon ZBCcomposite bodies, similar manufacturing steps are applicable to titaniumand hafnium parent metal composite bodies as well.

Broadly, in a first preferred embodiment, after forming a ZBC compositeaccording to the process disclosed in Application '533, the ZBCcomposite is embedded in a graphitic or carbon donor material bedding,which is contained in an appropriate refractory vessel. The filledrefractory vessel is heated in, for example, an electric resistancefurnace containing an argon atmosphere. During heating, it is believedthat small amounts of H₂ O or O₂ become available for reaction. Thesesmall amounts of H₂ O or O₂ are either intrinsically present in theargon gas or are liberated from the graphite bedding material or the ZBCcomposite. Thus, upon heating, carbon in the graphitic bedding materialcan react with oxygen to form a gaseous carburizing species. It also ispossible to provide a direct source of a carburizing species, such as,for example, a CO/CO₂ mixture or a H₂ /CH₄ mixture. It is theorized thatcarbon from the carburizing species dissolves into the ZrC_(1-x) phasein the ZBC composite and the carbon can then be transported throughoutthe ZBC composite by a vacancy diffusion mechanism. Thus, carbon can betransported so as to contact the residual parent metal to formadditional amounts of a parent metal-carbide phase (e.g., if zirconiumis the parent metal, the phase ZrC_(1-x) results due to the carburizingtreatment). However, some carbon from the graphite bedding material mayalso be directly diffused into the ZrC_(1-x) phase.

Likewise, in a second preferred embodiment, after forming a ZBCcomposite according to the process disclosed in Application '533, theZBC composite is embedded in a bedding comprising B₄ C which iscontained, for example, in a graphite crucible. The crucible is heatedin a suitable vacuum furnace which is evacuated and backfilled with aninert gas, preferably argon. The furnace is heated and maintained at atemperature sufficient to permit a reaction between any unreactedzirconium in the ZBC composite and the B₄ C bedding. During heating, itis believed that the B₄ C reacts with zirconium to form additional ZrB₂.Due to the boride formation or boriding, the mechanical properties ofthe composite can be modified. For example, as the volume percent ofresidual or unreacted zirconium parent metal decreases, the fracturetoughness decreases. A similar relationship has been discovered inreference to 4-point bending strengths of ZBC composites that have beenborided. However, due to the conversion of residual parent metal, thehigh temperature strength of the composite body increases. Accordingly,by subjecting a ZBC composite to a boriding process, the mechanicalproperties of the composite can be tailored to provide a wide range ofdesired properties in the final product.

In a third preferred embodiment, after forming a ZBC composite accordingto the process disclosed in Application '533, the ZBC composite may benitrided. Specifically, a ZrC-ZrB₂ -ZrN composite may be formed. A ZBCcomposite may be embedded in a nitrogen donor material, for example,ZrN, which may be contained in a refractory vessel. The refractoryvessel, such as Al₂ O₃, is heated, for example, in an electricresistance heated vacuum furnace. During heating, it may be desirablefor nitrogen gas to pass through the furnace. It is believed that theformation of ZrN is a diffusion controlled process wherein a source ofnitrogen diffuses in the bulk of the ZBC to react with any residualzirconium in the ZBC composite. As zirconium nitride is formed, it isbelieved the carbide phase dissolves into the nitride to form Zr(C_(x)N_(1-x))_(y). Accordingly, by subjecting a ZBC composite to a nitridingprocess, the mechanical properties of the composite can be tailored toprovide a wide range of desired properties in the final product.

Such post-treatment processing is advantageous because it permitsconversion of a residual parent metal phase into, for example, a harderand more refractory phase. Specifically, in applications which requirehigh temperature strength, a ZBC composite begins to lose strength at atemperature at or above the melting point of the residual parent metalphase. By post-treating the ZBC composite by a carburization process, aboriding process and/or a nitriding process, the parent metal phase isconverted into a carbide, a boride and/or a nitride of the parent metal(e.g., Zr parent metal is converted to ZrC, ZrB₂ and/or ZrN,respectively). The amount of parent metal which typically remains in aZBC composite produced according to the method in Application '533 isabout 5-40 volume percent. Upon exposing the ZBC composite to apost-treatment process, the amount of residual zirconium parent metalremaining can be reduced to, for example, about 0 to about 2 volumepercent.

The modified ZBC composite is useful for aerospace components such asnozzle inserts because the low metal content permits the ZBC compositeto be used in even higher temperature applications than previouslythought possible, without significantly compromising the fracturetoughness and thermal shock resistance of the ZBC composite body. Thus,the post-treatment process of the present invention is particularlyapplicable for applications which require a resistance to hightemperature erosion, have good thermal shock properties, and have arelatively high elevated temperature strength at a temperature of, forexample, 2200°-2700° C.

Moreover, because each post-treatment process is time-dependent, apost-treatment zone or surface (e.g., a carburized, borided or nitridedzone or region) can be created on a ZBC composite body. Thus, anexterior surface of the ZBC composite body can be made to bewear-resistant, while the ZBC composite core retains a high metalcontent having a corresponding high fracture toughness. Such a ZBCcomposite body would be particularly applicable in the manufacture ofwear plates, wear rings, and impeller inserts for various corrosive anderosive industrial pump applications. Specifically, zirconium metal hasa very high corrosion resistance to strong acids, but the metal, byitself, has poor wear characteristics. Thus, by modifying a ZBCcomposite body, a wear-resistant ceramic outer surface can be formulatedwith a corrosion-resistive composite interior. Moreover, ifsubstantially all of the zirconium metal is transformed to a ceramicphase (e.g., a ZrC_(1-x) phase), and the post-treatment process iscontinued, it is possible to increase, for example, the carbon contentin the ZrC_(1-x) phase (e.g., from about ZrC₀.58 to about ZrC₀.96). Ifsuch conversion is induced to occur, then the hardness and refractoryproperties of the ZBC composite can be expected to increase.Substantially parallel analysis can be made to each of the boriding andnitriding post-treatment processes.

Thus, the present method, and the novel composite body producedtherefrom, even further expand the potential applications for ZBCcomposite bodies.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic elevational view in cross-section showing a ZBCcomposite body 3 embedded in a graphitic powder bedding 2 and containedwithin a refractory vessel 1, to be processed according to the presentinvention.

FIG. 2 is a photomicrograph at 1000× of a section of the compositeproduced according to Example 2.

FIG. 3 is a photomicrograph at 1000× of a section of the compositeproduced according to Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is based on the discovery that the properties of aceramic composite body, particularly a ceramic composite body which ismanufactured by reactive infiltration of a parent metal of zirconium,hafnium or titanium into a boron carbide mass, can be modified by apost-manufacturing treatment. Such post-manufacturing treatmentscomprise a carburization process, a boriding process, and/or a nitridingprocess. Each of the above-mentioned post-manufacturing treatments canalter the microstructure, and thus the resultant mechanical properties,of a portion or substantially all of a ZBC composite body.

In a first preferred embodiment, a ZBC composite body, producedaccording to Application '533 (discussed above herein), can be modifiedby exposing the composite to a gaseous carburizing species. Such agaseous carburizing species can be produced by, for example, embeddingthe ZBC composite body in a graphitic bedding and reacting at least aportion of the graphitic bedding with moisture or oxygen in a controlledatmosphere furnace. However, the furnace atmosphere should comprisetypically, primarily, a non-reactive gas such as argon. The use of argongas from Matheson Gas Products, Inc., produces desirable results. It isnot clear whether impurities present in the argon gas supply thenecessary O₂ for forming a carburizing species, or whether the argon gasmerely serves as a vehicle which contains impurities generated by sometype of volatilization of components in the graphitic bedding or in theZBC composite body. In addition, a gaseous carburizing species could beintroduced directly into a controlled atmosphere furnace during heatingof the ZBC composite body.

Once the gaseous carburizing species has been introduced into thecontrolled atmosphere furnace, the lay-up should be designed in such amanner to permit the carburizing species to be able to contact at leasta portion of the surface of the ZBC composite body buried in the looselypacked graphitic powder. It is believed that carbon in the carburizingspecies, or carbon from the graphitic bedding, will dissolve into theinterconnected zirconium carbide phase, which can then transport thedissolved carbon throughout substantially all of the ZBC composite body,if desired, by a vacancy diffusion process. The diffusion of carbon intothe residual zirconium parent metal is quite low. Thus, absent thezirconium carbide phase, it would not be practical, or economical, toattempt to dissolve carbon throughout all of the residual zirconiummetal in the ZBC composite body, because the process would take aninordinate amount of time. In this regard, the diffusion of carbon inthe zirconium carbide phase and in the zirconium metal phase are bothtime dependent. However, the rate of transport of carbon in thezirconium carbide phase is much faster than the transport rate of carbonin the zirconium metal phase. Once a desirable amount of carbon has beendiffused into the ZBC composite body and contacts residual zirconiumparent metal, the zirconium parent metal is converted into ZrC. Suchconversion is desirable because the modified ZBC composite will have anincreased hardness and an increased elastic modulus, at the limitedexpense of both flexural strength and toughness. Moreover, the elevatedtemperature properties will also improve because of a lower metalcontent in the ZBC composite. It has been discovered that ZBC compositeshaving a residual parent metal in an amount between 5 to 30 volumepercent can be modified by a post-carburization treatment to result inabout 0 to about 2 volume percent, typically about 1/2 to about 2 volumepercent, of parent metal remaining in the ZBC composite body. Thus,substantially all of the parent metal, however, typically about 41/2 to28 volume percent of the parent metal, can be transformed from zirconiuminto ZrC.

Moreover, by controlling the time of exposure of the ZBC composite bodyto any one of the post-manufacturing treatments discussed, namely, thecarburizing, boriding, and/or nitriding and controlling the temperatureat which these treatment processes occur, a modified zone or layer canbe formed on at least one exterior surface of a ZBC composite body. Suchpost-treatment processes can result in a hard, wear-resistant surfacesurrounding a core of ZBC composite material having a higher metalcontent and higher fracture toughness.

In summary, it has been found that by subjecting a ZBC compositecontaining, typically between about 5-30 volume percent of residualzirconium parent metal, to a carburizing, a boriding, and/or anitriding, species in a controlled atmosphere furnace operating at atemperature of about 1500°-2200° C., for a period of time of about 5-48hours, a modified ZBC composite will be formed resulting in a moredesirable composite body.

The following are examples of the present invention. The examples areintended to be illustrative of various aspects of a post-manufacturingtreatment of a composite body, particularly a ZBC composite body.However, these examples should not be construed as limiting the scope ofthe invention.

EXAMPLE 1

A ZBC composite body formed according to Example 1 disclosed inApplication '533 was produced. Table 1 shows various mechanicalproperties of the formed ZBC composite body. All surfaces of the ZBCcomposite body were degreased ultrasonically by using acetone andethanol. The ZBC composite was then buried in a high purity graphitepowder bedding having an average particle diameter of about 75 microns.The graphite powder was purchased from Lonza, Inc., and was identifiedas KS-75. The graphite powder bedding was contained within a graphitemold (Grade ATJ from Union Carbide). The mold was covered on a topsurface thereof with a graphite cover plate. The complete assembly ofthe buried ZBC composite body was then placed into a closed atmosphereresistance heating furnace. The atmosphere in the furnace was argon fromMatheson Gas Products, Inc. The furnace was first evacuated at roomtemperature to a pressure of 1×10⁻⁴ Torr and thereafter backfilled withargon. The furnace was then evacuated to a pressure of about 1×10⁻² Torrand thereafter heated to a temperature of about 500° C. under vacuum.The furnace was again backfilled with argon which then remained flowingat a rate of about one liter per minute and was maintained at a pressureof about 2 psi. The furnace was heated to a temperature of about 1750°C. over a 6-hour period and then held at 1750° C. for about 12 hours.The furnace was then cooled for about 6 hours. After cooling, thecarburized ZBC composite was removed from the furnace and any excessgraphite powder was removed by grit blasting.

Table 1 shows the mechanical properties of the ZBC composite after thecarburization treatment had been effected. It is evident that the amountof residual zirconium parent metal was reduced from about 10% to about1/2%, by volume; the hardness, elastic modulus, and shear modulus allincreased. However, the increase occurred at the limited expense offlexural strength. It is noted that a flexural strength of about 500 MPais adequate for many aerospace applications.

                  TABLE 1                                                         ______________________________________                                                      Before   After                                                                Carburization                                                                          Carburization                                          ______________________________________                                        Zr Content, vol %                                                                             9.9        0.5                                                                80.6 HRA   81.9 HRA                                           Hardness        1011 HK    1388 HK                                            Elastic Modulus, GPa                                                                          364        442                                                Shear Modulus, GPa                                                                            158        184                                                Flexural Strength                                                                             875        497                                                MPa (4-point)                                                                 ______________________________________                                    

While the present invention has been disclosed in its preferredembodiments, it is to be understood that the invention is not limited tothe precise disclosure contained herein, but may otherwise be embodiedin various changes, modifications, and improvements which may occur tothose skilled in the art, without department from the scope of theinvention as defined in the appended claims.

EXAMPLE 2

A preform comprising B₄ C was formed by mixing about 477 grams of 1000grit B₄ C and about 9.5 grams of Dow XUS 40303 binder and about 715grams of methylene chloride which was sediment cast into a 7 inchdiameter ATJ graphite mold. Before sediment casting, the graphite moldwas sanded with a relatively coarse grit sandpaper. The preform wasplaced into a furnace in order to burnout or remove the binder. Thefurnace was then evacuated and backfilled with argon. During thesubsequent heating step, argon was passed through the furnace at a rateof approximately 2 liters per minute. The furnace was heated from roomtemperature up to about 200° C. in about four hours. This temperaturewas maintained for approximately two hours. The furnace was heated fromabout 200° C. to about 350° C. at a rate of approximately 20° C. perhour. The temperature was increased from about 350° C. to about 450° C.in about two hours. The furnace was permitted to cool to roomtemperature in approximately eight hours. The preform weighed about 466grams and measured about seven inches in diameter and about 0.6 inchesin thickness.

A nuclear grade zirconium sponge weighing about 2333.25 grams suppliedby Western Zirconium was cleaned and air dried at about 45° C. for onehour and at 70° C. for at least two hours. The zirconium sponge wasplaced directly on top of the B₄ C preform inside the graphite mold. Thegraphite mold was placed on top of a 10×10×4 inch inverted AGSX boatinto an electric resistance vacuum chamber furnace. The furnace wasevacuated and backfilled with argon. A vacuum was drawn on the furnaceand the furnace was brought to a temperature of about 1000° C. After1000° C. was reached, argon at 2 liters/min was passed through thefurnace having a chamber pressure of about 2 psi. Heating was continueduntil a temperature of about 1900° C. was reached. The total time toreach 1900° C. was about 8.5 hours. This temperature was maintained forapproximately one hour. The furnace was permitted to cool to roomtemperature in about 12 hours. The graphite crucible was removed fromthe furnace and inspected. It was discovered that the zirconium spongehad reactively infiltrated the B₄ C to form a platelet reinforcedcomposite comprising zirconium diboride and zirconium carbide.

The platelet reinforced composite weighed approximately 2670 grams. Thecomposite was lightly sand blasted in order to remove unreacted B₄ C.After the sand blasting treatment, the composite weighed approximately2570 grams and measured approximately 7 inches in diameter and about oneinch in thickness. The formed composite then was subjected to a boridingtreatment.

Specifically, the above described platelet reinforced composite wasembedded in 1000 grit B₄ C in a graphite crucible having an innerdiameter of approximately 8 inches. The amount of B₄ C utilized weighedapproximately 521 grams and was obtained from ESK. The graphite cruciblecontaining the platelet reinforced composite and the B₄ C beddingmaterial was placed into a vacuum furnace. The furnace was evacuated andbackfilled with argon. The furnace was heated at a rate of approximately300° C. per hour. When a temperature of about 1000° C. was reached,argon was passed through the furnace at a rate of approximately 2 litersper minute. The chamber pressure was about 2 psi. The furnace wascontinually heated until a temperature of about 1900° C. was obtained.This temperature was maintained for about 30 hours. The furnace waspermitted to cool to room temperature at a rate of approximately 200° C.per hour. The graphite crucible was removed and inspected. It wasdiscovered that the B₄ C bedding had reacted with residual zirconiummetal in the ZBC platelet reinforced composite. The borided compositehad a reduced metal content on the order of about 0-2 volume percent.

FIG. 2 is a photomicrograph at 1000× of a section of the modified ZBCcomposite produced according to the method of Example 2. The darkerregions are platelets of ZrB₂. The gray region comprise ZrC.

EXAMPLE 3

A ZBC body was formed substantially according to the procedures setforth in Example 1 in Application '533. All surfaces of the ZBCcomposite were degreased and ultrasonically cleaned by utilizing acetoneand ethanol. The ZBC body weighed approximately 3.6 grams and wasembedded in 1.0-5.0 micron ZrN powder which was contained in an Al₂ O₃refractory boat. The Al₂ O₃ boat containing the ZrN powder and the ZBCbody was placed into an electric resistance tube furnace. The furnacewas evacuated and backfilled with dried nitrogen gas. During subsequentheating steps, nitrogen was passed through the furnace at a rate ofapproximately 300 CC per minute. The furnace was heated at a rate ofapproximately 200° C. per hour until a temperature of about 1600° C. wasreached. This temperature was maintained for about 12 hours. The furnacewas cooled at a rate of approximately 200° C. per hour. The aluminacrucible was removed from the furnace and inspected. It was discoveredthat a nitrogen species had reacted with the ZBC body to form a ZrNphase.

FIG. 3 is a photomicrograph at 1000× of a section of the modified ZBCcomposite produced according to the method of Example 3. The darkestareas correspond to platelets of ZrB₂. The dark region in the upper lefthand side, which is defined by ZrB₂ platelets, comprises ZrC₂. Thelighter region on the lower left-hand side comprises Zr(C_(x)N_(1-X))_(y). The lightest region comprise Zr metal.

We claim:
 1. A method of producing a self-supporting body comprisingproducing a first composite by:selecting a parent metal; heating saidparent metal in a substantially inert atmosphere to a temperature aboveits melting point to form a body of molten parent metal and contactingsaid body of molten parent metal with a mass comprising boron carbide;maintaining said temperature for a time sufficient to permitinfiltration of molten parent metal into said mass and to permitreaction of molten parent metal with said boron carbide to form at leastone boron-containing compound; continuing said infiltration reaction fora time sufficient to produce said first composite which comprises aself-supporting body comprising at least one parent metalboron-containing compound; and subjecting said first composite to apost-treatment process which at least partially reduces the amount ofresidual parent metal in said first composite, thereby forming saidself-supporting body.
 2. A method of producing a self-supporting bodycomprising producing a first composite by:selecting a parent metal;heating said parent metal in a substantially inert atmosphere to atemperature above its melting point to form a body of molten parentmetal and contacting said body of molten parent metal with a masscomprising boron carbide; maintaining said temperature for a timesufficient to permit infiltration of molten parent metal into said massand to permit reaction of molten parent metal with said boron carbide toform at least one boron-containing compound; continuing saidinfiltration reaction for a time sufficient to produce said firstcomposite which comprises a self-supporting body comprising at least oneparent metal boron-containing compound; and subjecting said firstcomposite to at least one post-treatment process comprising a processselected from the group consisting of exposing said first composite to aboriding environment and exposing said first composite to a nitridingenvironment, which results in at least a partial conversion of theresidual parent metal to a corresponding ceramic in said firstcomposite, thereby forming said self-supporting body.
 3. The methodaccording to claim 2, wherein said self-supporting body compriseszirconium carbide and zirconium diboride.
 4. The method according toclaim 1, wherein said parent metal comprises at least one metal selectedfrom the group consisting of hafnium, titanium, and zirconium.
 5. Themethod according to claim 2, wherein said parent metal comprises atleast one metal selected from the group consisting of hafnium, titanium,and zirconium.
 6. The method according to claim 1, further comprisingreducing said amount of residual parent metal to about 0-2.0% by volume.7. The method according to claim 2, further comprising converting asufficient amount of residual parent metal so that the amount ofresidual parent metal in said self-supporting body comprises about0-2.0% by volume.
 8. The method according to claim 4, wherein saidparent metal comprises a zirconium sponge.
 9. The method according toclaim 5, wherein said parent metal comprises a zirconium sponge.
 10. Themethod according to claim 1, further comprising heating said firstcomposite body to a temperature of about 1500°-2200° C. for a period ofabout 5-48 hours.
 11. The method according to claim 1, furthercomprising post-treating said first composite body in a mannersufficient to produce a modified zone surrounding at least a portion ofsaid self-supporting body.
 12. The method according to claim 2, furthercomprising post-treating said first composite body in a mannersufficient to produce a modified zone surrounding at least a portion ofsaid self-supporting body.
 13. The method according to claim 1, whereinsaid self-supporting body comprises zirconium carbide and zirconiumdiboride.